Polymeric membrane based separation processes such as reverse osmosis, pervaporation and perstraction are conventional. In the pervaporation process, a desired feed component, e.g., an aromatic component, of a mixed liquid feed is preferentially absorbed by the membrane. The membrane is exposed at one side to a stream comprised of a mixture of liquid feeds and a vacuum is applied to the membrane at the opposite side so that the adsorbed component migrates through the membrane and is removed as a vapor from the opposite side of the membrane via a solution-diffusion mechanism. A concentration gradient driving force is therefore established to selectively pass the desired components through the membrane from its upstream side to its downstream side.
The perstraction process is utilized to separate a liquid stream into separate products. In this process, the driving mechanism for the separation of the stream into separate products is provided by a concentration gradient exerted across the membrane. Certain components of the fluid will preferentially migrate across the membrane because of the physical and compositional properties of both the membrane and the process fluid, and will be collected on the other side of the membrane as a permeate. Other components of the process fluid will not preferentially migrate across the membrane and will be swept away from the membrane area as a retentate stream. Due to the pressure mechanism of the perstraction separation, it is not necessary that the permeate be extracted in the vapor phase. Therefore, no vacuum is required on the downstream (permeate) side of the membrane and permeate emerges from the downstream side of the membrane in the liquid phase. Typically, permeate is carried away from the membrane via a swept liquid.
The economic basis for performing such separations is that the two products achieved through this separation process (i.e., retentate and permeate) have a refined value greater than the value of the unseparated feedstream. Membrane technology based separations can provide a cost effective processing alternative for performing the product separation of such feedstreams. Conventional separation processes such as distillation and solvent extraction can be costly to install and operate in comparison with membrane process alternatives. These conventional based processes can require a significant amount of engineering, hardware and construction costs to install and also may require high levels of operational and maintenance personnel costs to maintain the associated facilities in an operating status. Additionally, most of these processes require the heating of the process streams to relatively high temperatures in order to separate different components during the processing steps resulting in higher energy costs than are generally required by low-energy membrane separation processes.
In general, the membrane technology in the present art has the benefit of lower per unit energy costs per volume of separation than the conventional technologies in present art. However, a major obstacle in perfecting the commercial operation of membrane separation technologies is to improve the flux and selectivity characteristics of the current membrane systems in order to make the construction costs and separation efficiencies of membrane technologies economically viable, for example, on a refinery scale operations and on-board vehicle separation processes.
A myriad of polymeric membrane compositions have been developed over the years. Such compositions include polyurea/urethane membranes (U.S. Pat. No. 4,914,064); polyurethane imide membranes (U.S. Pat. No. 4,929,358); polyester imide copolymer membranes (U.S. Pat. No. 4,946,594); polyimide aliphatic polyester copolymer membranes (U.S. Pat. No. 4,990,275); and diepoxyoctane crosslinked/esterfied polyimide/polyadipate copolymer (diepoxyoctane PEI) membranes (U.S. Pat. No. 5,550,199).
These copolymeric membranes are generally comprised of “soft segments” and “hard segments” which form polymer chains in the membrane. The soft segments of the polymer provide the active area for the selective diffusion of the permeate through the membrane. However, these soft segments of the membrane have limited structural and thermal strength characteristics. Therefore, in order to provide structural strength to the membrane, a hard segment polymer (e.g., the reaction product of a dianhydride and a diamine) is added to the soft segment polymer in a suitable solvent to form long copolymer chains in the final membrane preferably comprised of alternating soft and hard polymer segments. These hard segments provide significant mechanical and thermal stability to the membrane, but are essentially non-permeable to the process stream components. These copolymer membranes of the prior art then undergo a high temperature “thermal cross-linking” to further promote molecular bonding between these copolymer chains in the final membrane composition.
For a given polymeric membrane composition, the flux across a given membrane is generally inversely proportional to the thickness of the membrane. Therefore, the cross-section of a constructed membrane is commonly very thin (on the order of about 0.1 to about 50 microns) in order to derive the selectivity benefit of the membrane while maximizing the flux characteristics of the membrane. However, for a membrane operated at constant feed composition and process conditions, the selectivity of a particular membrane composition is substantially independent of the thickness of the membrane and is principally dependent upon the compositional characteristics of the membrane.
Therefore, in order to increase the selectivity for membrane processes, new membrane compositions must be discovered that have improved selectivity characteristics. Although a high flux capacity of a membrane is desired, deficiencies in a membrane composition's flux characteristics can be overcome by increasing the active membrane area or fabricating membranes of thinner cross sections. Similar “mechanical variables” generally cannot be utilized to improve a membrane composition's inherent selectivity performance.
Additionally, some of the compounds that are utilized in the present art for the fabrication of polymeric membranes are categorized as toxic to human life and/or harmful to the environment. In particular, two of these compounds of concern, methylene dianiline (“MDA”) and methylene diochloroaniline (“MOCA”), are used in the fabrication of the polyethylene adipate (PEA)/diepoxy based membranes in the prior art (see U.S. Pat. Nos. 4,990,275 and 5,550,199 to Ho et al.). The Sigma-Aldrich chemical company website classifies MDA as toxic, a possible carcinogen, and harmful to aquatic environments. Sigma-Aldrich, Chemical Name 4,4′-Methylene-13C-dianiline <http://www.sigmaaldrich.com/catalog/search/ProductDetail/ALDRICH/491500> (accessed May 8, 2006). The Sigma-Aldrich chemical company website classifies MOCA as toxic, a possible carcinogen, and extremely harmful to aquatic environments. Sigma-Aldrich, Chemical Name 4,4′-Methylene-bis(2-chloroaniline), <http://www.sigmaaldrich.com/catalog/search/ProductDetail/ALDRICH/117323>, (accessed May 8, 2006).
The use of hazardous components in the fabrication of polymeric membranes also results in higher costs of manufacturing due to increased shipping and handling costs as well as higher costs for the installation, operation and maintenance of personnel and environmental protective equipment required for the manufacture and handling of these compounds. Therefore, in addition to the potential health concerns and potential adverse environment aspects of utilizing hazardous components, there is also is an economically driven need in the industry associated with the discovery and utilization of new non-hazardous materials for the fabrication of membranes which can meet or exceed the processing capabilities, durability and performance of the polymeric membranes of the prior art.
Therefore there is a need in the industry for new membrane compositions with improved inherent selectivity characteristics. There is also a separate need in the industry for new membrane compositions utilizing materials which possess non-toxic and minimal negative environmental impact properties.